I personally think they were at a stalemate. Obi-Wan saw an "out" by jumping onto the land, I'm not sure if he expected Anakin to follow, but he did say "don't do it." The book doesn't really clear this up at all, as it happens faster than in the movie.

When AniVader says "This is the end for you my master." I think he was assuming that once he got into close range with no readily available escape/retreat for Obi-wan that AniVader would eventually get in a killing blow. He didn't anticipate the reverse flip onto "shore."

As RS said, they were simply getting nowhere. And Obi-Wan, most likely, knows that in the long run - Anakin is simply stronger than him. There's very little, outside of Anakins flip over Obi Wan and landing behind him, that says that Kenobi was losing the fight. But it is made clear that in raw power, Anakin has the edge. Whether Anakin's contineously backing him down, twisting his arm or choking him.

I think in the novel (haven't read it....yet), Obi-Wan also senses Sidious approaching Mustafaaaaaaaaarrrr, so he has to call it quits.

Perhaps he doesn't feel merciful enough to finish off Ani, but he CERTAINLY has time. Cmon, he could have killed him in like 2 seconds and left. Obi-wan and Sidious' ships never came close to each other...

Why can Obi-wan sense Sidious coming? If he could, why couldn't Yoda sense Obi-wan and AniVader fighting and go even the odds?

I think the only reason Obi-wan was able to hang with AniVader is that he was emotional, but able to control it. Not let it effect his actions. Do a jump flip at an opponent holding higher ground or something. Given time, Ani's pure power would defeat him.